1. Field of the Invention
The present invention relates to an optical scanner which is capable of altering the diameter of an image-formation spot and the scanning range on a surface to be scanned, and to a system for an image recording device or the like, using this optical scanner.
2. Related Background Art
An example of an optical scanning optical system which is mainly applied to a system for an image recording device or for an image reading device, and which can alter the diameter of a light spot on a surface to be scanned is disclosed in Japanese Laid-Open Patent Application Nos. 55-155328 and 60-26924. Such an optical scanning system is constructed as follows: as shown in FIG. 7, a portion, in which a scanning light beam becomes parallel to a light beam emitted from a light source, is provided between the light source and a deflecting device, such as a polygonal mirror, for scanning the light beam emitted from the light source. Converting the diameter of the light beam in that portion by an optical means, such as an afocal zoom expander, allows a spot diameter on an image-formation surface to be altered.
In addition to the above construction, another construction is also well known in which a light limiting means, such as an aperture stop having a variable diameter, is provided between a light source and a deflecting device for scanning a light beam emitted from the light source so as to convert the diameter of the light beam.
In the above-mentioned constructions, the diameter of the light beam emitted from the light source is converted by the optical means (the zoom expander or the aperture stop) arranged between the light source and the deflecting device. The light beam thus converted is scanned by the deflecting device, such as the polygonal mirror, thereby impinging upon an image-formation lens, such as an f.theta. lens. It then forms a light spot on the surface to be scanned, followed by scanning. For example, when the diameter of the light beam is converted to a small diameter, the spot diameter on the surface to be scanned becomes enlarged during an image formation on the image-formation lens, because the f-number grows substantially large. On the contrary, when the diameter of the light beam is converted to a large diameter, the spot diameter on the surface to be scanned is reduced.
In each of the above constructions, in brief, the diameter of the light beam is converted before the beam reaches the polygonal mirror. The diameter of the light beam impinging upon the image-formation lens, which is arranged between the polygonal mirror and the surface to be scanned, differs from the diameter of the light beam before the beam is converted. As a result, the spot diameter on the surface to be scanned can be converted.
However, it is not appropriate to use the above conventional means to obtain variable-powered images together with normal-sized images by altering the spot diameter for the following reasons.
As regards the above-mentioned former conventional example:
(1) In the device disclosed in the Japanese Laid-Open Patent Application No. 55-155328, since only the spot diameter of the light beam grows small, and since the scanning range as well as the scanning speed on the surface to be scanned remain unchanged, it is required to increase the picture element recording density in order to obtain reduced images. To this end, the frequency of a picture element clock must be altered to speed it up. The above causes the electrical system to be complicated, thereby placing on the device a large load, with the result that costs are raised.
(2) To avoid the problem in item (1), as disclosed in the above Japanese Laid-Open Patent Application No. 60-26924, it is possible to change the rotational speed of the polygonal mirror in accordance with the alternations in the spot diameter. A control system for this purpose, however, is complicated.
(3) If the diameter of the light beam is made wide so as to obtain a fine spot diameter, the wide light beam is deflected, and thus the polygonal mirror must be large-sized.
(4) The size of the image-formation lens, for forming the wide beam into an image on the surface to be scanned, becomes larger.
(5) Because the image-formation lens is used to form the wide light beam into the image, the aberration and the surface precision of the image-formation lens are strictly required.
As regards the latter conventional example:
In addition to the above problems, when the aperture stop narrows the diameter of the light beam to obtain a wide spot diameter, an Airy ring may be generated in a spot formed on the surface to be scanned, because of diffraction caused by the aperture stop. This results in a problem in that it is impossible to obtain good images.